In a single substrate processing tool, when a hot etching solution is dispensed onto a cold substrate, such as a semiconductor wafer, the etch rate of the substrate or a film on the substrate is initially low but gradually increases as the surface of the substrate is heated by the hot etching solution. This time variation in the etch rate is undesirable for several reasons. Firstly, since the hot etching solution is often dispensed near the center of a spinning substrate, the substrate surface is not heated uniformly in a radial direction. This results in a radial variation in the etch rate across the substrate surface, and thus radial variation in the overall amount of material that is etched from the substrate. Secondly, the etching solution is often an expensive high purity chemical (e.g., phosphoric acid), and since the etch rate during the initial heat-up period is low, a significant portion of the etching solution that is dispensed during the heat-up period is effectively wasted, and it performs less etching the etching solution that is dispensed after the substrate surface has reached the desired temperature.
Several ideas have been proposed in order to solve these problems. In one example, hot sulfuric acid is dispensed to the upper surface of the substrate in order to heat up the substrate before etching with phosphoric acid. The sulfuric acid may be atomized using steam to further increase the heating effect. Sulfuric acid can be heated to very high temperatures (about 200° C.), and it is significantly less expensive than phosphoric acid. The sulfuric acid can also potentially be reclaimed and reused to reduce the chemical cost. However, a problem with this method is that sulfuric acid cannot be used if the upper surface of the substrate has exposed films (e.g., SiGe films) that are etched or damaged by the sulfuric acid.
In another example, hot water is dispensed to the upper surface of the substrate. The hot water is commonly compatible with materials that are exposed on the upper surface on the substrate (e.g., SiGe), but the use of hot water but limits the pre-heating of the substrate to a temperature of about 90° C. If steam is used simultaneously with the hot water, a substrate temperature approaching 100° C. may be reached. However, a substrate temperature approaching 100° C. is still far short of what is needed for many important wet etching processes, such as silicon nitride etching using phosphoric acid. An attempt to use hot water and/or steam to pre-heat the substrate prior to dispensing a hot phosphoric etching solution would result in unacceptable radial non-uniformity, wasting of the etching solution, and long processing times.
Accordingly, new methods are required to address these and other problems encountered in high-temperature wet processing in single wafer tools.